System and method for power generation using a hybrid geothermal power plant including a nuclear plant

ABSTRACT

A hybrid geothermal power system is discussed. The system includes a geothermal system including power plant ( 101 ) and pumping station ( 102 ) and a nuclear plant ( 103 ). Pumping station ( 102 ) is used to inject fluid from reservoir ( 104 ) through an injection well ( 105 ) into the bedrock ( 106 ) (also referred to as the hot dry rock HDR zone) and extracted via a secondary bore (extraction well) usually coupled to the power plant ( 101 ). In the present example however the injection well is linked to the extraction well ( 107 ). As fluid is injected into the bedrock a drop in temperature occurs due to heat transfer to the fluid. Nuclear plant ( 103 ) is utilized to combat this drop, the plant ( 103 ) has the fissionable components ( 1091, 1092, 1093 ) of the reactor positioned within bores ( 1081, 1082, 1083 ) within the HDR zone.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No.PCT/AU2012/000850, International Filing Date Jul. 13, 2012, and whichclaims the benefit of AU patent application No. 2011902916, filed Jul.15, 2011, the disclosures of both applications being incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to power generation. Inparticular although not exclusively the present invention relates tosystems and methods for the production of power utilising geothermalenergy.

2. Discussion of the Background Art

Given the increased awareness of the need for controlling emissions ofCO₂ has seen considerable investment in clean/green technology. One ofthe largest sources of CO₂ emissions comes from the generation of powerand more from coal powered stations. There are presently are number ofpower generation technologies which have a significantly lower carbonfootprint to that of fossil fuel powered stations.

One such alternative is nuclear power, nuclear power provides about 6%of the world's energy and 13-14% of the world's electricity, with theU.S., France, and Japan together accounting for about 50% of nucleargenerated electricity. While nuclear power is a sustainable energysource that reduces carbon emissions, it is exceedingly controversial.As recent examples in Japan and those of Chernobyl and Three Mile Islandhave shown the threat of meltdown is an ever present concern.

Another concern with nuclear power plants is the production of nuclearwaste. A typical 1000-MWe nuclear reactor produces approximately 20cubic meters (about 27 tonnes) of spent nuclear fuel each year (but only3 cubic meters of vitrified volume if reprocessed). Spent nuclear fuelis initially very highly radioactive and so must be handled with greatcare and forethought.

However, it will decrease with time. After 40 years, the radiation fluxis 99.9% lower than it was the moment the spent fuel was removed fromoperation. Still, this 0.1% is dangerously radioactive. After 10,000years of radioactive decay, according to United States EnvironmentalProtection Agency standards, the spent nuclear fuel will no longer posea threat to public health and safety.

When first extracted, spent fuel rods are stored in shielded basins ofwater (spent fuel pools), usually located on-site. The water providesboth cooling for the still-decaying fission products, and shielding fromthe continuing radioactivity. After a period of time (generally fiveyears for US plants), the now cooler, less radioactive fuel is typicallymoved to a dry-storage facility or dry cask storage, where the fuel isstored in steel and concrete containers.

In addition to the problems of meltdown and waste there are alsosecurity concerns. Nuclear reactors and waste dumps are prime targetsfor terrorist, cause a meltdown and you can take out a large populatedarea and spread radioactive materials across a wider radius. The wasteitself is also a target as it can be used in the manufacture of dirtybombs etc.

An alternate approach to nuclear power is that of geothermal powergeneration. Electricity generation from geothermal power requires hightemperature resources that can only come from deep underground. The heatmust be carried to the surface by fluid circulation. This circulationsometimes exists naturally where the crust is thin: magma conduits bringheat close to the surface, and hot springs bring the heat to thesurface. Until recently most geothermal electric plants have been builtexclusively where high temperature geothermal resources are availablenear the surface. The development of binary cycle power plants andimprovements in drilling and extraction technology may enable enhancedgeothermal systems over a much greater geographical range.

Enhanced Geothermal Systems (EGS) are a new type of geothermal powertechnologies that do not require natural convective hydrothermalresources. Until recently, geothermal power systems have only exploitedresources where naturally occurring heat, water and rock permeability issufficient to allow energy extraction from production wells. However,the vast majority of geothermal energy within reach of conventionaltechniques is in dry and non-permeable rock. EGS technologies “enhance”and/or create geothermal resources in this hot dry rock (HDR) throughhydraulic stimulation.

When natural cracks and pores will not allow for economic flow rates,the permeability can be enhanced by pumping high pressure cold waterdown an injection well into the rock. The injection increases the fluidpressure in the naturally fractured rock which mobilizes shear events,enhancing the permeability of the fracture system. This process, termedhydro-shearing [3], used in EGS is substantially different fromhydraulic tensile fracturing used in the oil & gas industries.

Water travels through fractures in the rock, capturing the heat of therock until it is forced out of a second borehole as very hot water,which is converted into electricity using either a steam turbine or abinary power plant system. All of the water, now cooled, is injectedback into the ground to heat up again in a closed loop. EGS/HDRtechnologies, like hydrothermal geothermal, are expected to be baseloadresources which produce power 24 hours a day like a fossil plant.Distinct from hydrothermal, HDR/EGS may be feasible anywhere in theworld, depending on the economic limits of drill depth.

In either case the thermal efficiency of geothermal electric plants islow, around 10-23% because geothermal fluids are at a low temperaturecompared with steam from boilers. By the laws of thermodynamics this lowtemperature limits the efficiency of heat engines in extracting usefulenergy during the generation of electricity. HDR wells are expected tohave a useful life of 20 to 30 years before the outflow temperaturedrops about 10° C. and the well becomes uneconomic. If left for 50 to300 years the temperature will recover. This limited life span and theexpenses of drilling etc. makes the use of such power stationseconomically undesirable limiting their application.

Clearly it would be advantageous to provide a system and method forpower generation which has a relatively low carbon footprint and whichameliorates some problems associated with the aforementioned prior art.

SUMMARY OF THE INVENTION Disclosure of the Invention

Accordingly in one aspect of the present invention there is provided asystem for power generation the system including:

-   -   a geothermal system including at least one injection shaft and        at least one extraction shaft and wherein the extraction shaft        is connected to the injection shaft;    -   a nuclear system including at least one reactor the reactor        being positioned remote from the nuclear plant; and    -   wherein the injection and extraction shafts extend a        predetermined depth and the at least one reactor is positioned        within the area defined between the injection and extraction        shaft's base.

In yet another aspect of the present invention there is provided amethod for producing power said method including the steps of:

-   -   drilling an injection shaft to a predetermined depth;    -   drilling an extraction shaft to a predetermined depth such that        the extraction shaft is connected to the injection shaft;    -   positioning a reactor core in the area defined between the        injection and extraction shaft's base;    -   pumping fluid into the injection shaft under low pressure;    -   extracting vapour under high pressure from the extraction shaft;        and    -   maintaining utilising the reactor core a substantially constant        temperature differential within the area between the injection        and extraction shafts to promote the conversion of the fluid to        a high pressure vapour stream.

Suitably the geothermal system includes a power plant and a pumpingstation. Preferably the injection shaft and extraction shaft are drilledto a depth between 4400 m and 5000 m. The injection shaft and extractionshaft may be connected such that they form a substantially U shapedconduit between the power station and a pumping station.

Suitably the fluid is pumped into the top of the injection well at arelatively low pressure. The fluid may be feed in at a head pressure ofbetween approx 30-40 psi. The pressure on the fluid at the base of theconduit may be between 5000 to 6000 psi and may be at a temperaturebetween 450° C. to 500° C. The pressure on the fluid along the length ofthe extraction well may vary in pressure between 6000 to 3000 psi andmay vary in temperature between 500° C. to 200° C. Preferably the fluidis ejected from the extraction well as high pressure vapour stream. Thehigh pressure vapour stream is ejected from the extraction well at apressure between 2500 psi-3300 psi. Suitably the high pressure vapourstream is unitised to drive one or more turbines housed within the powerplant. The power plant may also contain one or more condensers forcondensing the vapour stream back into a fluid for re-injection backinto the injection well.

The geothermal system may include additional injection and extractionshafts positioned between the power plant and pumping station toincrease the generation capacity of the plant. Any suitably fluid havingan appropriate vaporisation temperature may be utilised in the system.Preferably the through the shafts is water.

Suitably the nuclear reactor is positioned in the region defined betweenthe injection and extraction shaft/s. Preferably the reactor ispositioned within a shaft at a depth of 3000 m to 4400 m. The reactormay be operated such that the heat generated by the fissionablecomponent is utilised to heat the surrounding area. Preferably thefissionable material is in the form of a set of uranium rods as used inmost standard nuclear reactors.

Suitably the shaft housing the reactor may include a series of explosivecharges placed at discrete points along its length. Preferably theexplosive charges are positioned such that when triggered they seal theshaft. The system may also include additional fail safe procedures suchas filing the shaft with a shielding a material to a predetermineddepth.

The nuclear system may include a plurality of reactors positioned withthe region between the injection and extraction shaft/s. The nuclearsystem may also be utilised to generate power directly from thereactor/s. In such instances a number of auxiliary shafts could beprovided to heat water to steam utilising the reactors in a conventionalmanner.

BRIEF DETAILS OF THE DRAWINGS

In order that this invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings, which illustrate preferred embodiments of the invention, andwherein:

FIG. 1 is a schematic diagram depicting a power generation systemaccording to one embodiment of the present invention; and

FIG. 2 is a schematic diagram depicting one possible construction of areactor core for use in a power generation system.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to FIG. 1 there is illustrated one arrangement of powergeneration system 100 according to one embodiment of the presentinvention. As shown the system is a hybrid geothermal system andincludes a geothermal system including power plant 101 and pumpingstation 102 and a nuclear plant 103.

The geothermal system in this instance operates in a similar manner tothat of most standard enhanced geothermal systems. More specifically thepumping station 102 is used to inject fluid from reservoir 104 throughan injection well 105 into the bedrock 106 (also referred to as the hotdry rock HDR zone). The injection well usually ranges in depth to from4000 m to 5000 m. Under most enhanced geothermal systems, however, thefluid injected into the crystalline bedrock 106 is extracted via asecondary bore (extraction well) usually coupled to the power plant 101.In the present example however the injection well is linked to theextraction well 107.

The advantage of the construction is that it obviates the need forhydraulic fracturing as occurs in the case of most enhanced geothermalsystems which can lead to seismic events. As shown the fluid (e.g.water) is pumped into the top of the injection well at a relatively lowpressure e.g. approx 30 psi to the base of the shaft which is at adistance of around 5000 m. At this depth the water pressure andtemperature radically increase e.g. the water pressure may be around6000 psi and at a temperature of approximately 500° C.

The continuous injection of fluid and the pressure exerted on the waterforce it up the extraction well 107. As the fluid rises up though theextraction well 107 both the temperature and pressure drops, in thiscase the water drops to a pressure of approximately 3500 psi around 3000m and temperature of 450° C. at this point the water beings to enter itsgaseous state (i.e. beings turning to steam). The water is ejected fromthe extraction well as high pressure (approx 3000 psi) steam which isthen used to drive the turbines of the power plant 101 to produceelectricity. While the geothermal system has been describe as utilisinga single injection and extraction shaft, it will of course beappreciated by those of skill in the art that additional shafts may alsobe utilised to increase the generation capacity of the plant.

As noted above one of the problems associated with geothermal systems istheir efficiency. The overall efficiency of the system degrades as heatis extracted from the HDR zone. To combat this drop in temperature thepresent invention utilises nuclear plant 103. Unlike a conventionalnuclear plant the reactor/reactors of the plant 103 are constructedwithin a series of bores 108 ₁, 108 ₂, and 108 ₃. More specifically thefissionable components 109 ₁, 109 ₂, 109 ₃ of the reactor are positionedwithin the HDR zone. It will of course be appreciated by those of skillin the art that while the present example utilises just three reactorshafts that more shafts may be utilised depending on the size of theplant and the desire electricity output.

Instead of the heat generated by the fission of the reactor rods beingutilised to directly heat water to steam for power generation thereactors are used to heat the surrounding bedrock 106. The increase intemperature of the bedrock can improve the overall efficiency of the ofthe geothermal system as the extracted steam remains at a highertemperature for longer enabling greater utilisation of the jet before tomuch energy is dissipated. Additionally given the reactors are requiredto heat the bedrock directly they can be run at a higher temperaturethan would normally be the case in a normal reactor. This is due mainlyto the thermal mass of the rock that the reactors must heat i.e. therock required a greater amount of energy input to produce a rise intemperature of a few degrees.

One advantage to this construction is that it can effectively increasethe lifespan of a geothermal plant i.e. temperature drop is counteracted by injection of heat from the nuclear plant. In addition the risksassociated with the reactor are minimised. As the core fissionablematerial is positioned within a strata of the Earth's crust where thematerials are already radioactive the risk of radioactive contaminationis minimal. Moreover given the depth of the reactor were it to go intomelt down the resultant blast would have minimal impact as it iscontained deep underground.

In the case of meltdown the system may be fitted with furthersafeguards. For instance each of the bores containing the reactors couldhave a series of explosive charges placed at discrete points along theirlength. In the event of a meltdown or other such failure the explosivecould be set off to collapse the relevant bore over the reactor. Inaddition a part of the bore could then be filled with a shielding amaterial i.e. a suitable layer of concrete or other suitable materialsuch as synroc etc. Additional when the fuel rods are spent there is noneed to remove them from the shaft, the shaft can be simply sealed tocontain the waste in a layer of the Earth's crust which is alreadyradioactive.

As can be seen form the above discussion it is possible to utilisestandard construction namely a series of fuel and control rods with amoderator medium disposed there between to improve the efficiency ofgeothermal power generation system by dissipating the heat generatedfrom the reactor to the surrounding bedrock. Given the distance betweenthe reactor core and the control controls it can be somewhat difficultto maintain control over the reaction to ensure efficient fuel usage(i.e. ensure prolonged operation of the reactor before sealing of thereactor core is required). Consequently the applicant has considered anumber of alternate reactor designs to improve fuel usage and heattransference. More specially the applicant has consider the use of VeryHigh Temperature Reactors (VHTR)

One type of VHTR design which is considered to be suitable for use inthe present invention is that of a pebble bed reactor. This type ofreactor is claimed to be passively safe that is, it removes the need forredundant, active safety systems. As these reactors are designed tohandle high temperatures, they can cool by natural circulation and stillsurvive in accident scenarios, which may raise the temperature of thereactor to 1,600° C. In addition the design of such reactors allow forhigher thermal efficiencies than that of more traditional reactors.

Typically most pebble bed reactors include a core containing a pluralityof spherical fuel elements (pebbles). The pebbles are made of pyrolyticgraphite (which acts as the moderator), and they contain thousands ofmicro fuel particles called TRISO particles. These TRISO fuel particlesconsist of a fissile material uranium, thorium or plutonium surroundedby a coated ceramic layer of silicon carbide for structural integrityand fission product containment. In standard pebble reactors the core isencased in a concrete housing into which a cooling gas is thencirculated. In addition the spent fuel is typically drawn away from thebase of the core with new fuel injected into the top of the core. In thepresent case the removal of spent fuel is not possible nor is thecirculation of a cooling gas necessary as the heat from the reactor isutilized to raise the temperature of the surrounding bedrock.Consequently the design of the pebble bed reactor for the present systemhas required some modification.

One possible construction of the pebble reactor core 200 for use in theenhanced geothermal power generation system of the present invention isshown in FIG. 2. As shown the core includes a housing 201, the housingin this example is constructed from synroc or other suitable material.The housing is in this instance is generally cylindrical and sized tofit within the bore 108 housing the reactor. The base of the housing 201is sealed, the upper end of the housing is open to permit the insertionof the pebbles 205 to fuel the reactor. In the depicted example theupper end of the housing is sealed via the use of a closure 202 to sealthe core.

The closure in this instance includes an aperture 203 for connection ofan umbilical 204. In addition to the aperture 203 the closure alsoincludes lugs for the attachment of tethers to permit the positioning ofthe reactor core 200 to the appropriate depth within the bore 108. Theumbilical in this instance is utilised to carry a number of servicesincluding sensor equipment to monitor the reactor's operation e.g.temperature, radiation levels etc. The umbilical may also carry coolinglines to maintain the operation of the reactor at optimum levels forfuel consumption and heat transfer. It will of course be appreciated bythose of skill in the art that the umbilical 204 would be formed from anappropriate heat resistant material capable of tolerating temperaturesin excess of 500° C. and which has sufficient tensile strength to resistshearing forces etc. which it may be exposed to within the bore.

It will be appreciated by those of skill in the art that while a coolantmay be introduced into the reactor core to optimise operating life ofthe reactor core it is not strictly necessary. In the present case thereactor core is able to run at higher temperatures for a prolongedperiod given the large thermal mass it is required to heat.

While the above discussion of the reactor core contemplates filling thereactor with the fissionable materials prior to insertion into the bore,the potential for exposure and possible meltdown are somewhat increased.Accordingly the reactor core 200 could be lowered a safe distance intothe bore 108 before the fuel is added via the umbilical or other suchfiling tube.

It will also be appreciated by those of skill in the art that as thereis no need to remove the core form the bore once it is positioned at therequired depth. Once the fuel is depleted the bore 108 can be sealed anda new bore drilled for receipt of a new core. Consequently noradioactive debris is brought back to the surface with all waste beingcontained with the hot dry rock zone which in its self is inherentlyradioactive.

In other embodiments of the invention the reactors could also beutilised to generate electricity in a conventional manner to supplementthe electricity generated from the geothermal plant. In such instancesthe closure or the reactor may be fitted with heat exchanger which whenthe closure is fitted would be retained within the reactor core the tailends of the exchange could then be coupled to a fluid inlet and outletlines.

It is to be understood that the above embodiments have been providedonly by way of exemplification of this invention, and that furthermodifications and improvements thereto, as would be apparent to personsskilled in the relevant art, are deemed to fall within the broad scopeand ambit of the present invention described herein.

The invention claimed is:
 1. A system for power generation the systemincluding: a geothermal system including a power plant, at least oneinjection shaft extending a predetermined depth to enable injection offluid to a hot dry rock (HDR) zone and at least one extraction shaftextending a predetermined depth to enable extraction of fluid from theHDR zone to be used by the power plant to generate power; and a nuclearsystem including a nuclear plant and at least one reactor, the reactorbeing positioned remote from the nuclear plant, wherein the at least onereactor is positioned within a region of the HDR zone between theinjection and extraction shafts to heat rock in the region of the HDRzone.
 2. The power generation system of claim 1 wherein the geothermalsystem includes a pumping station.
 3. The power generation system ofclaim 1 wherein the injection shaft and extraction shaft are drilled toa depth between 4400 m and 5000 m.
 4. The power generation system claim2 wherein the injection shaft and extraction shaft are connected.
 5. Thepower generation system claim 4 wherein the injection shaft andextraction shaft are connected to form a substantially U shaped conduitbetween the power plant and the pumping station.
 6. The power generationsystem of claim 1 wherein the fluid is pumped into the top of theinjection shaft at a low pressure.
 7. The power generation system ofclaim 6 wherein the fluid is fed by the pumping station into theinjection shaft at a pressure between 30-40 psi.
 8. The power generationsystem of claim 1 wherein the fluid is extracted from the extractionshaft as a high pressure vapour stream.
 9. The power generation systemof claim 8 wherein the pressure on the fluid along the length of theextraction shaft varies in pressure between 3000 to 6000 psi.
 10. Thepower generation system of claim 8 wherein the high pressure vapourstream is utilised to drive one or more turbines housed within the powerplant.
 11. The power generation system of claim 10 wherein the powerplant further includes one or more condensers for condensing the vapourstream back into a fluid for re-injection back into the injection shaft.12. The power generation system of claim 1 wherein the geothermal systemincludes additional injection and extraction shafts positioned betweenthe power plant and pumping station.
 13. The power generation system ofclaim 1 wherein the nuclear reactor is positioned within a shaft at adepth of 3000 m to 4400 m.
 14. The power generation system of claim 13wherein the rector is powered via a set of uranium rods.
 15. The powergeneration system of claim 13 wherein the shaft housing the reactorincludes a series of explosive charges placed at discrete points alongits length, wherein triggering of the explosive charges seals the shaft.16. The power generation system of claim 1 wherein the nuclear systemincludes a plurality of reactors positioned with the region between theinjection and extraction shafts.
 17. The power generation system ofclaim 16 wherein each reactor within the plurality of reactors is apebble bed reactor.
 18. The power generation system of claim 17 whereineach of the reactors includes a reactor core coupled to the nuclearplant via an umbilical, and wherein at least one sensor is coupled tothe reactor core via the umbilical.
 19. The power generation system ofclaim 17 wherein the umbilical provides for the passage of a coolant tothe reactor core.
 20. A method for producing power said method includingthe steps of: drilling an injection shaft to a predetermined depth toenable injection of fluid to a hot dry rock (HDR) zone; drilling anextraction shaft to a predetermined depth to enable extraction of fluidfrom the HDR zone to be used by a power plant to generate power;positioning a reactor core in a region of the HDR zone between theinjection and extraction shaft to heat rock in the region of the HDRzone; pumping fluid into the injection shaft under low pressure;extracting vapour under high pressure from the extraction shaft;maintaining, utilising the reactor core, a substantially constanttemperature differential within the region of the HDR zone between theinjection and extraction shafts to promote the conversion of the fluidto a high pressure vapour stream, and generating power by a power plantusing the vapour extracted from the extraction shaft.